The sedimentation rate -- or “sed rate,” for short -- is a blood test that checks for inflammation in your body. It’s one clue for your doctor that you might have a disease linked to inflammation, like arthritis or cancer.
The sed rate test measures how fast red blood cells fall to the bottom of a tube. Inflammation creates proteins that make red blood cells fall more quickly.
Another name for this test is erythrocyte sedimentation rate (ESR). Red blood cells are called erythrocytes. Sedimentation is the process by which they fall to the bottom of the tube.
Why You Might Get a Sed Rate
Your doctor might order the sed rate test if you have symptoms like these:
- Stiff joints
- Pain in your shoulders, neck, or pelvis
- Appetite loss
- Weight loss without trying
The sed rate test can be part of the process of discovering if you have one of these conditions:
- Giant cell arteritis (swelling in the lining of your blood vessels)
- Lupus (an autoimmune disease that damages the skin, joints, and other parts of your body)
- Polymyalgia rheumatica (causes stiff and painful muscles)
- Rheumatoid arthritis (autoimmune disease in which the immune system attacks your joints)
- Systematic vasculitis (inflammation in your blood vessels)
- Temporal arteritis (inflammation of blood vessels in your head, most often the artery that runs under the scalp on the side of the forehead)
You might also get this test once you've started treatment for one of these conditions. The sed rate can help your doctor see how well your body is responding to treatment.
Taking the Blood Sample
You don’t need to do anything special to prepare. It's just a basic blood test.
A nurse or other health care provider will take a sample of your blood, usually from a vein in your arm. He will first tie a band around the upper part of your arm to make your vein fill with blood and swell up. Then he’ll clean the area with an antiseptic, and place a needle into your vein. Your blood will collect into a vial or tube.
The process should only take a couple of minutes. Afterward, you’ll get a piece of gauze and a bandage over the area to stop the bleeding.
You may feel a slight sting as your blood is drawn. Afterward, you may have a small bruise. You might feel feel dizzy and sore, and there might be some bleeding.
The Results and What They Mean
Your sample will go to a lab. You should have the results in 1 or 2 hours.
A lab technician will place your red blood cells into a tall, thin tube and check how far they fall in 1 hour. When you have inflammation in your body, abnormal proteins in your blood make red blood cells form into clumps. These clumps are heavy, so they fall to the bottom of the tube more quickly than single blood cells.
The faster the blood cells sink, the more inflammation you have in your body.
The sed rate test reports in millimeters (mm) the distance between the clear liquid (plasma) at the top of the tube and your red blood cells after 1 hour. The normal range is:
- 0 to 15 mm/hour in men younger than 50
- 0 to 20 mm/hour in men older than 50
- 0 to 20 mm/hour in women younger than 50
- 0 to 30 mm/hour for women older than 50
A high sed rate is a sign you have a disease that causes inflammation in your body.
Some conditions and medicines can affect the speed at which red blood cells fall, and they may affect your test results. These include:
Other Tests You May Need
The sed rate test can only tell your doctor that you have inflammation somewhere in your body. It can't show where the inflammation is or what caused it. Your doctor will use the sed rate test together with other tests of inflammation, such as C-reactive protein, to make a diagnosis.
Talk to your doctor about the results of your sed rate test, and any other tests you have. Make sure you understand what the results mean, and how they'll affect your treatment.